With recent great improvement in optical communication and multimedia fields, development toward a highly information-intensive society has been accelerated. Accordingly, an optoelectronic device using conversion of a photon into an electron or vice versa has been emphasized in modern information electronic industries.
The semiconductor optoelectronic device is classified into an electroluminescent device, a light receiving device and combinations thereof.
Most displays fabricated up to date are of a light-receiving type, whereas an electroluminescent display has self-luminous characteristics and thus can exhibit a fast response and high luminance, without the need for a backlight. Thus, the electroluminescent display is regarded as a next-generation display.
The electroluminescent device is divided into inorganic luminescent devices and organic luminescent devices, depending on the kind of light emitting layer material.
Organic electroluminescence (EL) means that energy produced when an electron and a hole transferred from a cathode and an anode, respectively, are combined in an organic material by an electric field applied to the organic material is emitted as light. Such electroluminescence of the organic material was reported by Pope et al., 1963. Since a multilayered luminescent device having quantum efficiency of 1% and luminance of 1000 cd/m2 at 10 V or less has been fabricated with the use of a colorant having p-conjugated structure of alumina-quinone, by Tang et al. of Eastmann Kodak, 1987, much research is being conducted. This device is advantageous because various materials can be easily synthesized according to a simple synthesis path, and color tuning is easy. However, processibility or heat stability is low, and also, upon the application of voltage, Joule heat generated from the light emitting layer causes rearrangement of molecules, thus negatively affecting the luminous efficiency or service life of the device. Therefore, an organic electroluminescent device having a polymer structure, capable of alleviating the above problems, is proposed.
In this regard, FIG. 1 shows a conventional organic electroluminescent device including substrate/anode/hole transport layer/light emitting layer/electron transport layer/cathode.
As shown in FIG. 1, an anode 12 is formed on a substrate 11. On the anode 12, a hole transport layer 13, a light emitting layer 14, an electron transport layer 15 and a cathode 16 are sequentially formed. As such, the hole transport layer 13, the light emitting layer 14 and the electron transport layer 15 are an organic thin film made of an organic compound. The organic electroluminescent device having the above structure is actuated as follows:
When voltage is applied to the anode 12 and the cathode 16, the hole injected from the anode 12 is moved to the light emitting layer 14 through the hole transport layer 13. Meanwhile, the electron is injected into the light emitting layer 14 from the cathode 16 through the electron transport layer 15, and the carriers are recombined in the region of the light emitting layer 14, to produce excitons. The excitons are changed from an excited state to a ground state, whereby a fluorescent molecule in the light emitting layer emits light, to form an image.
Organic materials used for the formation of organic films of EL devices may be of low molecular weights or high molecular weights.
Where low-molecular weight organic materials are applied, they can be easily purified to an impurity-free state, and thus is excellent in terms of luminescence properties. However, low-molecular weight materials do not allow inkjet printing or spin coating, and are of poor heat resistance such that they are deteriorated or recrystallized by the heat generated during the operation of the device.
On the other hand, in the case of using high molecular weight materials (i.e., polymer), an energy level is divided into a conduction band and a valance band, as wave functions of p-electrons present in its backbone overlap with each other. The band gap between the conduction band and the valence band defines the semiconductor properties of the polymer and thus, control of the band gap may allow the visualization of full colors. Such a polymer is called a p-conjugated polymer.
The first development of an EL device based on the conjugated polymer poly(p-phenylenevinylene) (hereinafter referred to as ‘PPV’) by a research team led by Professor R. H. Friend, Cambridge University, England, 1990 has stimulated extensive active research into organic polymers of semiconductor properties. In addition to being superior to low-molecular weight materials in heat resistance, polymeric materials can be applied to large-surface displays by virtue of their ability to be inkjet printed or spin coated. PPV and polythiopene (Pth) derivatives in which various functional moieties are introduced are reported to be improved in processability and exhibit various colors. However, such PPV and Pth derivatives, although applicable for emission of red and green light at high efficiency, have difficulty in emitting blue light at high efficiency. Polyphenylene derivatives and polyfluorene derivatives are reported as blue light-emitting materials. Polyphenylene is of high stability against oxidation and heat, but of poor luminescence efficiency and solubility.
As with the polyfluorene derivatives, the relevant prior arts are as follows:
U.S. Pat. No. 6,255,449 discloses 9-substituted-2,7-dihalofluorene compounds, and oligomers and polymers thereof, which are suitable as luminescent materials, e.g., light emitting or carrier transport layers in light emitting diodes.
U.S. Pat. Nos. 6,309,763 and 6,605,373 disclose an elecroluminescent copolymer containing a fluorine group and an amine group in the repeating unit. According to the '763 patent, such a copolymer is useful as light emitting layer or hole transport layer in the electroluminescent device.
WO 02/77060 discloses a conjugated polymer containing spirobifluorene units. According to this reference, the polymer as disclosed therein shows an improved property profile as electroluminescent material in electronic components such as PLED.